Energy-curable coating compositions, especially glossy coatings such as produced by crosslinking polymerization of monomers by actinic and non-actinic radiation (ultraviolet, electron beam, gamma, or microwave radiation) are used primarily for their ability to provide a protective layer with high gloss, typically above 85% (60° gloss) at 2 microns thickness. Numerous sectors of graphic arts, especially the packaging industry, require that in addition to coatings having high gloss, such coatings also must have a low coefficient of friction (“COF”), in the order of 0.3 and below, or a slide angle below 15°. The COF and slide angle are measured by placing two surfaces in contact with each other and measuring the ratio of the forces to start or continue movement of one surface onto the other.
Two major alternatives are known in the art to bring about low COF in energy-curable compositions. Soluble additives, such as silicone or fluorinated surfactants may reduce COF as well as dispersed solid or semi-solid materials, such as waxes. However, both of the additives have major disadvantages.
The most efficient coating additives are typically soluble in the coating mixture, therefore providing stable transparent coatings with the highest possible gloss. For example, in U.S. Pat. No. 4,886,551 alkoxylated silicones are disclosed as coating additives. While careful selection of silicones can actually improve gloss of base coating formulas and they may lower the coating's COF, they also exhibit major inconveniences such as migration into the package contents (food, juices, milk, etc), migration and then contamination of rollers and other areas of the printing press or unwanted areas of the print such as flaps, hot sealing areas, and others, and also not being re-writable nor gluable. As a result, silicones are undesirable in many industrial printing processes. It is also known in the art that non-migrating silicones are typically much less efficient in electron beam coatings than in ultra-violet (“UV”) coatings to reduce COF. Even with excessive amounts of silicone, a plateau in COF is reached that cannot be overcome by further addition of surfactants.
In order to reach the lowest COF, waxes and silicones are often combined together. The waxes known in the art for such use are insoluble particulates of varying size and shape. They can be of animal origin (i.e. beeswax), vegetable (i.e. carnauba), synthetic ((i.e. polyolefins, PTFE) or derived from petroleum ((i.e. paraffins). Certain waxes also increase scuff resistance and other surface properties of coatings. The major drawback of all waxes is the dramatic impact on coating gloss. Typical waxes show a linear reduction in 60° gloss of about 10 points per percent used. Multiple attempts have been made by wax producers to overcome this feature through control of the wax shape, average particle size and breadth of distribution, and surface modifications, all leading to rather small effects. Compounding in a liquid carrier (energy curable or not) is today the most efficient way of obtaining some gloss retention with conventional wax products.
In paper web manufacturing and sizing, for example, one known alternative to common waxes are water-repellent insoluble metal salts of fatty acids. Such salts are obtained from the reaction of a fatty acid preferably selected from a group of stearic, oleic, linoleic, or palmitic acids, combined with a metal that is a member of Group I, II or III on the periodic table. For example, fatty acid derivatives are described as adequate dispersants for PTFE in non-aqueous systems, such as petroleum distillates and solvent-based and powder coatings. U.S. Pat. Nos. 5,863,875 and 6,066,601 for instance, describe lubricant compositions (concentrates) containing in a suitable solvent the combination of fatty amides which are made with multifunctional amines and monoacids in order to obtain non-crosslinked amide, and at least one fluoropolymer.
The use of waxes in combination with a hydrocarbyl succinamic acid or amine or ammonium salt thereof has been described in GB 1,465,175 as useful for improving the low temperature flowability of petroleum middle distillate fuels but no mention is made in this field of the effect of such compounds on the tribology of such oils.
Multiple patents describe aqueous lubricant dispersions containing calcium stearate and a fatty acid-derived lubricant component, either non-ionic such as polyethylene glycol mono-esters of fatty acids (U.S. Pat. No. 2,425,828), ethylene oxide adducts of fatty amides, sorbitan esters of fatty acids, ethylene oxide and propylene oxide adducts of sorbitan esters of fatty acids, lower alkyl mono ethers of polyethylene glycol mono esters of fatty acids (U.S. Pat. No. 4,659,589), or anionic such as water-soluble ammonium, substituted ammonium or alkali metal salt of sulfated fatty acid and esters, fatty acid soaps, and sulfated glycerides of unsaturated fatty acids (U.S. Pat. No. 4,676,836). Exhaustive references to the existing art can be found in U.S. Pat. No. 5,527,383. In U.S. Pat. No. 5,356,544, stearic acid salts with alkali metal oxide or hydroxide including ammonia produced in-situ are put in presence of calcium oxide, forming a water dispersion of calcium stearate with controlled particle size and without any other surfactant. However, none of these materials are described to be used outside of water-based paper sizing compositions.
Alternates to calcium stearate are also described, using fatty acids and derivatives as a dispersed phase. In U.S. Pat. No. 4,766,015 a lubricant for paper coatings is described comprising a fatty acid, a surfactant and a crude lecithin component as the dispersed phase. In U.S. Pat. No. 5,858,933, surfactantless compositions are described, using fatty esters instead of acids, providing superior lubrication during the paper coating application. These lubricant compositions are again described as merely being dispersable in water or water-based paper coating compositions.
Thus, there exists a need for a wax-free composition for use in energy-curable coating compositions to reduce the COF while still maintaining and improving gloss of the coating.